The present invention relates to a single-vision spectacle lens to correct eyesight and particularly, to an astigmatic-power lens having a cylindrical power to correct astigmatism of an eye.
An astigmatic-power lens has a cylindrical power to correct astigmatism of an eye. In a conventional astigmatic-power lens, a front surface (an object side) is a spherical surface or a rotationally-symmetrical aspherical surface and a back surface (an eye side) is processed to be a toric surface to have a difference between the refractive powers in two orthogonal directions in a plane perpendicular to an optical axis, thereby correcting astigmatism.
However, in the above-described conventional astigmatic-power lens, since the rotationally-asymmetrical component to add the cylindrical power is given by the toric surface, a good transmitting optical performance is given in the directions of a first principal meridian along which the minimum surface refractive power is obtained and a second principal meridian along which the maximum surface refractive power is obtained, while a sufficient transmitting optical performance cannot be given in other directions between the first and second principal meridians.
It is therefore an object of the present invention to provide an astigmatic-power spectacle lens, which is capable of having a sufficient optical performance in any direction even if the lens has a cylindrical power to correct astigmatism of an eye.
For the above object, according to the present invention, there is provided an improved single-vision astigmatic-power spectacle lens to correct eyesight, which includes:
a front surface; and
a back surface,
wherein at least one of the front and back surfaces is a rotationally-asymmetrical aspherical surface that has a rotationally-asymmetrical component to correct the aberrations in the directions between first and second principal meridians of the rotationally-asymmetrical surface caused by adding the cylindrical power.
With this construction, since at least one of front and back surfaces is a rotationally-asymmetrical surface, the aberration caused by adding the cylindrical power can be corrected.
Further, the rotationally-asymmetrical surface may further include an another rotationally-asymmetrical component to add the cylindrical power for correcting astigmatism of an eye.
When the one surface has two rotationally-asymmetrical components, the other surface may be a rotationally-symmetrical surface, preferably, a spherical surface.
Further, in order to respond to various combinations of spherical power, cylindrical power and cylindrical axis direction, it is desirable that semifinished lens blanks whose front surfaces are finished are stockpiled and a back surface of the selected semifinished lens blank is processed according to the customer""s specification in order to shorten delivery times.
The present invention may be also defined as follows. Namely, when a sag z(h, xcex8) of the rotationally-asymmetrical surface at a point (h, xcex8) with respect to an x-y plane is expressed as a function of the angle xcex8 while fixing the distance h, the curve of the function has a larger gradient in close to the local maximum and a smaller gradient in close to the local minimum as compared with the curve interpolated by the sine curve in any distances h within the range of 10xe2x89xa6hxe2x89xa620.
The z-axis is a normal to the rotationally-asymmetrical surface at a framing reference point that is the origin of the x-y-z coordinate system and is coincident with a pupil position of a user when the spectacle lens is installed on a frame.
The x-axis is coincident with a first principal meridian of the rotationally-asymmetrical surface along which the minimum surface refractive power is obtained.
The y-axis is coincident with a second principal meridian of the rotationally-asymmetrical surface along which the maximum surface refractive power is obtained.
Symbol h is a distance from the origin in the x-y plane, and xcex8 is an angle of the line passing the origin and the point (h, xcex8) with respect to the x-axis in the x-y plan.
Further, according to another definition, the present invention is characterized in that the following condition (1) is satisfied in any distances h within the range of 10xe2x89xa6hxe2x89xa620:
z(h,45) less than {f(h)+g(h)}/2xe2x80x83xe2x80x83(1)
where
f(h) is the sag z(h, 0) on the x-axis, and
g(h) is the sag z(h, 90) on the y-axis.
When the spectacle lens is a negative lens, it is desirable that the following condition (2) is satisfied in any distances h within the range of 10xe2x89xa6hxe2x89xa620:
xe2x88x920.00010 less than [z(h,45)xe2x88x92{f(h)+g(h)}/2]/[{f(h)xe2x88x92g(h)}xc3x97hxc3x97CYL] less than xe2x88x920.00008xe2x80x83xe2x80x83(2)
where
CYL (unit: Diopter) is a cylindrical power.